The present invention relates to a carbon fiber reinforced material and to a method for making it, and more particularly relates to such a carbon fiber reinforced material and to a method for making it, in which, particularly, reinforcing carbon fibers which are embedded in a matrix metal are oriented therein in a closed loop configuration.
In the prior art, the concept of utilizing carbon fibers as reinforcing material for a composite material, such as one utilizing carbon fibers embedded in a matrix of matrix metal, has been per se well known. Therefore, there have been proposed various types of carbon fiber reinforced material and methods for making the same. For example, in the specifications of Japanese Patent Applications Serial Nos. Sho. 60-59414 (1985), Sho. 60-49415 (1985), and Sho. 60-59416 (1985), as well as in the specification of Japanese Patent Laying Open Publication Ser. No. Sho. 60-243239 (1985), none of which is it intended hereby to admit as prior art to the present patent application except to the extent in any case required by applicable law, and all of which are or relate to patent applications filed by an applicant the same as the entity assigned or owed a duty of assignment of the present patent application, there are disclosed such carbon fiber reinforced materials and various method for making them. Since carbon fibers have a relatively low coefficient of thermal expansion, and due to the relatively high rigidity and the relatively light weight of carbon fibers, such carbon fiber reinforced composite materials are endowed with various desirable mechanical properties. For example, in the particular case that such a carbon fiber reinforced composite material is utilized for making at least a part of a piston in an internal combustion engine, the thermal expansion of the piston is kept desirably low, and improved rigidity is ensured at the same time as precluding excessive thermal expansion of the bearing surfaces. And, further, it has been per se conceived of, and practiced, to orient such reinforcing carbon fibers, within the matrix material such as matrix metal in which they are embedded, in a closed loop configuration. Thus, various materials incorporating reinforcing carbon fibers oriented in various closed loop configurations such as annlar ring shaped configurations or cylindrical configurations or the like have already been proposed and practiced, as have processes for manufacturing them.
Now, in general the characteristics of carbon fibers vary quite significantly according to the disposition of the carbon atoms that make them up, i.e. according to the so called degree of graphitization and the so called degree of crystallization thereof. In general, it is per se known that the Young's modulus of carbon fibers increases according to increased graphitization of said carbon fibers, while on the other hand the moistenability of said carbon fibers with a typical matrix metal and their reactability with such a typical matrix metal are correspondingly decreased along with such increased graphitization thereof. In many cases, such as those in which the matrix metal which it is desired to use for the composite material is a metal which has a comparatively high reactivity with carbon fibers such as aluminum alloy or magnesium alloy, it is desired to reduce the intensity of the reaction between the carbon fibers and the matrix metal. In such a case, therefore, carbon fibers which have a relatively high degree of graphitization, and which consequently have a relatively high value for their Young's modulus such as for example 40 ton/mm.sup.2, (40,000 Kg/mm.sup.2) are typically used.
There is however a problem with fiber reinforcement of a composite material with such a relatively high Young's modulus type of carbon fibers having a relatively high degree of graphitization, in that, when as is often desirable the reinforcing carbon fibers are oriented in a closed loop configuration such as an annular ring shaped configuration or a cylindrical configuration or the like as described above, and when as is generally the case the composite material is manufactured by the pressurized casting method or at least is manufactured by some method in which the matrix metal in the liquid phase is commingled with the carbon fibers and penetrates into the interstices between the carbon fibers which are oriented in said closed loop configuration, then the finished product is found to be very prone to cracking and fissurization of the reinforcing carbon fiber material, and it is very difficult in practice to ensure a good quality for the resulting composite material.